I'm in the top floor and attic of an 1890s building. There's been a fair bit of settling in the past and the floors run off from the back of the building to the front. The joists have also sagged in the middle of the rooms. To bring them up to straight and (almost) level, some bits will need to be raised by almost 5".

Whats the best way to go about dealing with this?

Should I scribe and cut new 2" wide bits to be fixed on top of each joist? I imagine this would be very time consuming and hard to get exactly right, but it would mean that I wouldn't need to rake out all the old horsehair plaster and other building muck that the original builders stuffed between all the joists for sound deadening between floors. This would be a major plus because it would also mean that there would be a much smaller chance of accidentally damaging the old ceilings beneath the floors I'm going to be working on.

Or should I just drill and bolt new 'joists' on to the sides of the old ones? This would be much quicker and easier to get just right, but like I say I'd have to rake out tons of muck, there'd be the chance of damaging the ceilings beneath, and I'd also be worried about the additional weight of the extra wood just causing more sagging. Also that drilling through the original joists might weaken them.

Can anyone offer me any advice please? The original joists are 2" thick and 9" deep, and the length of them across the rooms I'm working on is about 10-12'.

A new floor independent of the old one? That would be nice! But apart from attaching new joists directly on to the sides of the old ones, how would I go about making them secure? The two rooms I am working in don't go as far out as the outer walls of the building so I don't have the opportunity to locate the ends of joists on top of supporting walls. I don't want to raise the floor at all, just to bring it back to what it should be...

Thanks welsh brickie - I think that would be my choice of what to do as well, if it weren't for the fact that the sag is 5" in places! I can't pack the joists up 5" and still expect to be able to nail down new floorboards on to them. That's why I was thinking of offering up new 2" wide joists alongside the old ones, levelling them up, scribing and cutting them to fit, then fixing them directly on top. But I don't know if that's even feasible. Is it? I don't mind 'time consuming', I just want something that will definitely work.

I've asked quite a few joiners and flooring companies to quote me for the work and so far nobody wants to touch it. Probably cause it's fiddly and awkward. I've been offered various unsatisfactory 'solutions' like packing the joists up a bit in the worst places to disguise the sag then sticking down a plywood base and laminate. But I want to do it properly. I'm fed up putting wedges under the front corners of all my bookcases so they don't tip forward! That's how bad the sag is. And I want to put down real floorboards...

Gypsy07,
If you have a 5" sag on a joist 9"x2" you have a serious overloading problem the span is just too great. Levelling the joists by packing will be a temporary solution. You need to find a way to provide additional support over the middle of the joists ie add a beam underneath them.
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